1. Field of the Invention
The present invention relates to apparatus for the measurement of the quantity of oxygen, or other relatively strong paramagnetic gas, in a gaseous mixture by use of its magnetic susceptibility through its effect on the induced motion of a test body in a magnetic field.
2. The Prior Art
The measurement of oxygen via its paramagnetic susceptibility has been known since the middle of the 19.sup.th century when Faraday showed that all materials interacted with a magnetic field. Gases were found in general to be repelled by a magnetic field and we described as being diamagnetic, whilst oxygen and some other gases were found to be attracted to a magnetic field and called paramagnetic. The very high paramagnetism of oxygen enabled its measurement by magnetic susceptibility techniques.
Two principal methods were originally developed for implementation of this technique. One was to employ a uniform magnetic field whilst the other employed the non-uniform field as originally described by Faraday. An example of the former technique is given by L. G. Gouy in Compt. Rend. Vol. 109 (1885) 935 while an example of the latter is given by P. W. Selwood in Magnetometry 2nd Edition, 1993, Interscience N/Y London 1956. The bulky and delicate nature of these instruments led to the development of further apparatus amongst which the most successful were those based on the original Faraday gas susceptibility balance. In these designs a test body of well defined shape is suspended inside a gas cell. The test body is made of materials, such as quartz, which have a low value of diamagnetic susceptibility and is suspended such that a strong non-uniform magnetic field is present across it. When the paramagnetic gas enters the test cell the gas tends to move to congregate at the point of strongest magnetic field causing a change in the position of the test body which can be measured and related to the concentration of paramagnetic gas in the cell.
Several forms of test body have been investigated including the commonly used dumbbell (e.g. G. C. Haven, Physical Review Vol. 41 (1932) 337) and also with modifications using a flattened structure (e.g. U.S. Pat. No. 3,815,018). Some other patents using dumbbell based designs for Faraday balances include U.S. Pat. No. 2,416,344 and U.S. Pat. No. 2,962,656. The apparatus described by these and other patents are similar in that all of them employ an optical method to detect the rotation of the dumbbell as the magnetic nature of the gas in the test cell is altered. Although in some cases a feedback system was employed in order to produce a null balance system, by electromagnetism, by electrostatic means (U.S. Pat. No. 3,026,472) and by variation of the magnetic field (U.S. Pat. No. 3,879,658) all of these techniques still required the use of optical means to provide the final measurement.
Non optical detection systems have also been disclosed that employ extra components to detect the rotation of the test body. These include the use of magneto-resistive pickup (GB-A-1 220 413), employment of separate high frequency excitation and pickup coils (U.S. Pat. No. 3,714,557), and capacitive sensing of the rotation, (U.S. Pat. No. 3,612,991). In these cases the use of an optical detection method has been replaced by the use of an alternative but in each case the new measurement technique is an additional system to the basic susceptibility balance and acts as an alternative to the optics.
WO92/05436 described an alternative arrangement for the measurement of the proportion of a paramagnetic gas not requiring the additional detection system extra to the basic susceptibility balance. In particular, it proposed that a test element should be made to vibrate in a varying magnetic field by the application of a current through the test element and where the electrical conductors used to apply the current to cause the current to cause oscillation were also used to sense the subsequent oscillation of the elements in the magnetic field. As described in that document, certain parameters of the oscillation, such as damping and frequency, are dependent on the proportion of the paramagnetic gas in the cell and therefore this proportion can be determined from measurements of the ongoing oscillation.